Dinosaur Fossil Examples

The appendicular skeleton of sauropods was characterized by the following:

■ A large coracoid attached to the scapula, with a widened anterior end.

■ Radius and ulna shorter than the humerus, and a femur 10-20% longer than the length of the humerus (with the notable exception of brachiosaurids).

■ Denser limb bones in general.

■ Large astragalus coupled with a smaller calcaneum (the latter absent in diplodocids), where the astragalus fits snugly with the tibia.

■ Pes with five digits and unguals on digits I to III.

■ Manus with five digits and smaller phalanges on digits II to V.

■ Ungual on digit I of manus (with the exception of a few Cretaceous species in which it is missing).

A gradual change in the cranial anatomy of sauropods that became more apparent through their evolutionary history was a migration of their nares from the anterior part of the skull to farther away from the mouth. This placed the nostrils of some sauropods on top of their heads (Fig. 10.4). This feature was once interpreted as an adaptation to an aquatic lifestyle, whereby nostrils on the highest point of the head aided in breathing while the majority of the body was submerged.

FIGURE 10.4 Skulls of the Late Jurassic diplodocids Apatosaurus (left) and Diplodocus (right) showing dorso-ventral positioning of nares in relation to the anterior portion of their skulls. Dinosaur National Monument, Vernal, Utah. Contrast with skull of Plateosaurus in Figure 10.3.

Independent evidence has since refuted this hypothesis (as discussed later), but the nares and their association with possible soft-tissue structures is still an area of speculation and controversy in sauropod research.

The cranial capacity of sauropods was rather limited: make a fist and it will approximate the size of the largest sauropod brain, no matter what its tonnage may have been. In fact, the cranial anatomy seemingly was a continuation of the evolutionary theme explored by prosauropods, in that it was simply a conduit for passing food to the gut and air to and from the lungs. Overall profiles of sauropod skulls can be broadly divided into:

1 rounded skulls with leaf-like (but thickened) teeth, as seen in brachiosaurids and camarasaurids; or

2 elongate skulls with pencil-like teeth, exemplified by diplodocids.

Teeth in any of the sauropods were typically non-serrated, as opposed to those of most prosauropods. Evidence from sauropod skulls relating to sensory abilities are not well defined, although some of them had well-developed orbits and nares.

Sauropods used to be classified into four major clades and a polyphyletic group: Diplodocidae, Brachiosauridae, Camarasauridae, Titanosauridae, and "Cetiosauridae." Recent cladistic analyses have resulted in a reclassification of these and other sauropod groupings. Node-based clades within this new classification include Eusauro-poda, Neosauropoda, Camarasauromorpha, Titanosauriformes, Lithostrotia, and Saltasauridae (Fig. 10.1). With this reorganization, Brachiosauridae and Titano-sauridae are now within Titanosauriformes, Camarasauridae is within Camarasauro-morpha, Diplodocidae is within Neosauropoda, and the formerly polyphyletic "Cetiosauridae" more likely constitutes a monophyletic one. In fact, Cetiosaurus, Barapasaurus (Lower Jurassic of India), and Patagosaurus (Middle Jurassic of Argentina) are regarded as members of this clade.

Neosauropoda includes the stem-based clade Diplodocoidea. Diplodocoidea is partially represented by diplodocids, probably the best known of sauropods. Diplodocids include familiar North American dinosaurs such as Apatosaurus, Barosaurus, Super-saurus, and Seismosaurus, and the eponymous Diplodocus (Fig. 10.5), all from the Late Jurassic. Cretaceous examples of diplodocoids include the Early Cretaceous Amargasaurus of Argentina (Fig. 10.6), Rebbachisaurus of Morocco, and Nigersaurus of Niger, as well as the Late Cretaceous Nemegtosaurus of Mongolia and Rayososaurus of Argentina.